This invention relates to magnetic separation and, more particularly, is concerned with a method of, and an apparatus for, separating magnetisable particles from a fluid containing them.
There are known apparatus, often referred to as wet magnetic separators, for separating a mixture of particles into a magnetisable fraction and non-magnetisable fraction. In such apparatus a slurry containing the mixture of particles is passed through a predetermined zone in which a magnetic field is established and the magnetisable particles, hereinafter referred to as the "native" magnetisable particles, are captured at collecting sites in the predetermined zone.
The force exerted on a spherical particle of magnetisable material in a magnetic field is given by the formula: EQU F = .chi. m (.pi. D.sup.3 /6) .multidot. H .multidot. dH/dx
where m is the volume magnetic susceptibility of the material, D is the diameter of the particle, H is the magnetic field intensity and dH/dx is the rate of change of the magnetic field intensity with distance. From this expression it can be seen that, if both the diameter D and the volume magnetic susceptibility .chi. m of the particles are small, it is necessary to provide a high intensity magnetic field and/or a magnetic field whose intensity changes rapidly with distance. Thus, in many known types of magnetic separators, the predetermined zone in which the magnetic field is established is packed with a porous magnetisable material which has a sufficiently open structure for the flow of slurry through it not to be unduly impeded and which still provides a large number of collecting sites of high magnetic field intensity so that a very non-homogeneous magnetic field is established. The porous magnetisable material may comprise, for example: a stack of corrugated or ridged plates; a filamentary material, such as steel wool, wire mesh or bundles of wires or fibres; a particulate material, such as spheres, pellets or particles of more irregular shapes such as iron filings; or a metallic foam such as can be made, for example, by electroplating carbon-impregnated foam rubber and then removing the rubber with a suitable solvent.
For a simple wet magnetic separator in which a paramagnetic particle of radius R and magnetic susceptibility .chi. in a fluid of viscosity .eta. moves with velocity V.sub.o relative to a ferromagnetic wire or radius a and a saturation magnetisation M.sub.s in a uniform magnetic field of intensity H.sub.o applied in a direction opposite to the direction of flow of the fluid, the longitudinal axis of the wire being oriented in a direction perpendicular to the direction of the magnetic field and to the direction of flow of the fluid, it can be shown mathematically that the chance of the paramagnetic particle being captured by the wire increases with the ratio V.sub.m /V.sub.o where V.sub.m is a quantity having the dimensions of speed and given by the expression: ##EQU1## Therefore, in order to maximise the number of native magnetisable particles captured by the wire without increasing the value of magnetic field intensity H.sub.o, it is necessary to minimise the value of V.sub.o. This relationship applies for magnetic separators utilising more complex magnetisable materials to separate a number of native magnetisable particles of differing size and differing magnetic susceptibilities from a fluid.